What is the ruling regarding my prayer and my friend's prayer, and is my prayer invalid, requiring repetition, because I prayed alone behind the Imam?

The Sunnah is for a solitary congregant to stand to the right of the Imam, based on the hadith of Ibn Abbas: "I prayed with the Prophet (peace be upon him) one night, and I stood to his left. So, the Messenger of Allah (peace be upon him) took hold of my head from behind me and placed me to his right, and he prayed."

As for the prayer of a congregant behind the row, it is valid but disliked by the majority of scholars if there is no excuse. The dislike is removed if there is an excuse.

Regarding the Imam's prayer, there is no disagreement concerning its validity.